1. Field of the Invention
The present invention relates to a zoom lens, which is adaptable to an image pickup apparatus using an image sensor, such as a digital still camera, video camera, TV camera, or monitoring camera.
2. Description of the Related Art
A photographic optical system used for an image pickup apparatus includes a zoom lens which should preferably provide the following features: short total lens length, compact size, wide angle of view, and high optical performance with a high zoom ratio. Such a photographic optical system also provides a focusing function. Therefore, it is also desired that a focus lens unit be compact and lightweight to attain high-speed and high-precision automatic focus.
It is important to appropriately set a zoom type, refractive power of each lens unit, and a lens configuration to downsize an entire zoom lens and to realize high-speed focusing with a compact and lightweight lens unit, while attaining a wide angle of view.
As a zoom lens of a negative lead type, in which a lens unit having negative refractive power is arranged closest to the object side, a five-unit zoom lens has been known. Examples are disclosed in U.S. Pat. No. 8,189,074, and U.S. Patent Application Publication No. 2010/0091170. In the five-unit zoom lens, a lens unit having negative refractive power, a lens unit having positive refractive power, a lens unit having positive refractive power, a lens unit having negative refractive power, and a lens unit having positive refractive power are arranged in order from an object side to an image side. In the five-unit zoom lens, a lightweight second lens unit having relatively a small diameter is regarded as a focus lens unit. When a lens unit formed by combining the second lens unit and a third lens unit is considered to be a lens unit having positive refractive power, a principal point position might be greatly shifted to the object side, compared to a second lens unit having positive refractive power in a general four-unit zoom lens in which a lens unit having negative refractive power, a lens unit having positive refractive power, a lens unit having negative refractive power, and a lens unit having positive refractive power are arranged. It is particularly difficult to secure back focus at the wide-angle end. The five-unit zoom lens described above also tends to have strong refractive power of each lens unit. Therefore, an appropriate refractive power arrangement for correcting aberration variation during zooming cannot be made, which makes it difficult to realize high performance and miniaturization.
A four-unit zoom lens has also been known, for example, from Japanese Patent Application Laid-Open No. 2003-131130, and Japanese Patent Application Laid-Open No. 2001-343584. In the four-unit zoom lens described above, a lens unit having negative refractive power, a lens unit having positive refractive power, a lens unit having negative refractive power, and a lens unit having positive refractive power are arranged in order from an object side to an image side. In the four-unit zoom lens described above, a compact and lightweight third lens unit moves during focusing. When the number of lenses in the focus lens unit is reduced to downsize the zoom lens, and the refractive power of the focus lens unit is set to be too strong, residual aberration of the focus lens unit might be large, so that aberration variation might increase during focusing.